Internet Engineering Task Force (IETF) A. Custura

Internet Engineering Task Force (IETF) A. Custura
Request for Comments: 9435 G. Fairhurst
Category: Informational R. Secchi
ISSN: 2070-1721 University of Aberdeen
July 2023

Considerations for Assigning a New Recommended Differentiated Services
Code Point (DSCP)

Abstract

This document discusses considerations for assigning a new
recommended Differentiated Services Code Point (DSCP) for a standard
Per-Hop Behavior (PHB). It considers the common observed re-marking
behaviors that the Diffserv field might be subjected to along an
Internet path. It also notes some implications of using a specific
DSCP.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9435.

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
2. Terminology
3. Current Usage of DSCPs
3.1. IP-Layer Semantics
3.2. DSCPs Used for Network Control Traffic
4. Re-marking the DSCP
4.1. Bleaching the DSCP Field
4.2. IP Type of Service Manipulations
4.2.1. Impact of ToS Precedence Bleaching
4.2.2. Impact of ToS Precedence Re-marking
4.3. Re-marking to a Particular DSCP
5. Interpretation of the IP DSCP at Lower Layers
5.1. Mapping Specified for IEEE 802
5.1.1. Mapping Specified for IEEE 802.1
5.1.2. Mapping Specified for IEEE 802.11
5.2. Diffserv and MPLS
5.2.1. Mapping Specified for MPLS
5.2.2. Mapping Specified for MPLS Short Pipe
5.3. Mapping Specified for Mobile Networks
5.4. Mapping Specified for Carrier Ethernet
5.5. Re-marking as a Side Effect of Another Policy
5.6. Summary
6. Considerations for DSCP Selection
6.1. Effect of Bleaching and Re-marking to a Single DSCP
6.2. Where the Proposed DSCP > 0x07 (7)
6.2.1. Where the Proposed DSCP&0x07=0x01
6.3. Where the Proposed DSCP <= 0x07 (7)
6.4. Impact on Deployed Infrastructure
6.5. Considerations to Guide the Discussion of a Proposed New
DSCP
7. IANA Considerations
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

The Differentiated Services (Diffserv) architecture has been deployed
in many networks. It provides differentiated traffic forwarding
based on the DSCP carried in the Diffserv field of the IP packet
header [RFC2474]. A common set of DSCPs are defined for both IPv4
and IPv6, and both network protocols use a common IANA registry
[DSCP-registry].

Diffserv associates traffic with a service class and categorizes it
into Behavior Aggregates (BAs) [RFC4594]. Configuration guidelines
for service classes are provided in [RFC4594]. BAs are associated
with a DSCP, which in turn maps to a Per-Hop Behavior (PHB).
Treatment differentiation can be achieved by using a variety of
schedulers and queues and also algorithms that implement access to
the physical media.

Within a Diffserv domain, operators can set Service Level
Specifications [RFC3086], each of which maps to a particular Per-
Domain Behavior (PDB) that is based on one or more PHBs. The PDB
defines which PHB (or set of PHBs) and, hence, for a specific
operator, which DSCP (or set of DSCPs) will be associated with
specific BAs as the packets pass through a Diffserv domain. It also
defines whether the packets are re-marked as they are forwarded
(i.e., changing the DSCP of a packet [RFC2475]).

Application -> Service
Traffic Class
|
Behavior -> Diffserv -> Per Hop
Aggregate Codepoint Behavior
|
Schedule,
Queue, Drop

Figure 1: The Role of DSCPs in Classifying IP Traffic for
Differential Network Treatment by a Diffserv Node

This document discusses considerations for assigning a new DSCP for a
standard PHB. It considers some commonly observed DSCP re-marking
behaviors that might be experienced along an Internet path. It also
describes some packet forwarding treatments that a packet with a
specific DSCP can expect to receive when forwarded across a link or
subnetwork.

The document is motivated by new opportunities to use Diffserv end-
to-end across multiple domains, such as [NQB-PHB], proposals to build
mechanisms using DSCPs in other standards-setting organizations, and
the desire to use a common set of DSCPs across a range of
infrastructure (e.g., [RFC8622], [NQB-PHB], [AX.25-over-IP]).

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

DSCPs are specified in the IANA registry [DSCP-registry], where a
variety of different formats are described. A DSCP can sometimes be
referred to by name, such as "CS1", and sometimes by a decimal, hex,
or binary value. Hex values are represented in text using prefix
"0x". Binary values use prefix "0b".

In this document, the symbol "&" denotes a bitwise AND of two
unsigned values.

3. Current Usage of DSCPs

This section describes the current usage of DSCPs.

3.1. IP-Layer Semantics

The Diffserv architecture specifies the use of the Diffserv field in
the IPv4 and IPv6 packet headers to carry one of 64 distinct DSCP
values. Within a given administrative boundary, each DSCP value can
be mapped to a distinct PHB [RFC2474]. When a new PHB is specified,
a recommended DSCP value among those 64 values is normally reserved
for that PHB and is assigned by IANA. An operator is not formally
required to use the recommended value; indeed, [RFC2474] states that
"the mapping of codepoints to PHBs MUST be configurable." However,
use of the recommended value is usually convenient and avoids
confusion.

The DSCP space is divided into three pools for the purpose of
assignment and management [DSCP-registry]. A summary of the pools is
provided in a table (where 'x' refers to a bit position with value
either '0' or '1').

DSCP Pool 1: A pool of 32 codepoints with a format of 0bxxxxx0, to
be assigned by IANA Standards Action [RFC8126].

DSCP Pool 2: A pool of 16 codepoints with a format of 0bxxxx11,
reserved for Experimental or Local (Private) Use by network
operators (see Sections 4.1 and 4.2 of [RFC8126].

DSCP Pool 3: A pool of 16 codepoints with a format of 0bxxxx01.
This was initially available for Experimental (EXP) Use or Local
Use (LU) but was originally specified to be "preferentially
utilized for standardized assignments if Pool 1 is ever exhausted"
[RFC2474]. Pool 3 codepoints are now "utilized for standardized
assignments (replacing the previous availability for experimental
or local use)" [RFC8436]. [RFC8622] assigned 0x01 from this pool
and consequentially updated [RFC4594]. Any future request to
assign 0x05 would be expected to similarly update [RFC4594].

Note that [RFC4594] previously recommended a Local Use of DSCP values
0x01, 0x03, 0x05, and 0x07 (codepoints with the format of 0b000xx1),
until this was updated by [RFC8436].

The DSCP space is shown in the following table. Each row corresponds
to one setting of the first three bits of the DSCP field, and each
column to one setting of the last three bits of the DSCP field.

+--------+------+---------+----+---------+----+---------+----+
| 56/CS7 | 57 | 58 | 59 | 60 | 61 | 62 | 63 |
+--------+------+---------+----+---------+----+---------+----+
| 48/CS6 | 49 | 50 | 51 | 52 | 53 | 54 | 55 |
+--------+------+---------+----+---------+----+---------+----+
| 40/CS5 | 41 | 42 | 43 | 44/VA | 45 | 46/EF | 47 |
+--------+------+---------+----+---------+----+---------+----+
| 32/CS4 | 33 | 34/AF41 | 35 | 36/AF42 | 37 | 38/AF43 | 39 |
+--------+------+---------+----+---------+----+---------+----+
| 24/CS3 | 25 | 26/AF31 | 27 | 28/AF32 | 29 | 30/AF33 | 31 |
+--------+------+---------+----+---------+----+---------+----+
| 16/CS2 | 17 | 18/AF21 | 19 | 20/AF22 | 21 | 22/AF23 | 23 |
+--------+------+---------+----+---------+----+---------+----+
| 8/CS1 | 9 | 10/AF11 | 11 | 12/AF12 | 13 | 14/AF13 | 15 |
+========+======+=========+====+=========+====+=========+====+
| 0/CS0 | 1/LE | 2 | 3 | 4 | 5 | 6 | 7 |
+========+======+=========+====+=========+====+=========+====+

Table 1: Currently Assigned DSCPs and Their Assigned PHBs

+----+----------------------+-----------+
| CS | Class Selector | [RFC2474] |
+----+----------------------+-----------+
| BE | Best Effort (CS0) | [RFC2474] |
+----+----------------------+-----------+
| AF | Assured Forwarding | [RFC2597] |
+----+----------------------+-----------+
| EF | Expedited Forwarding | [RFC3246] |
+----+----------------------+-----------+
| VA | Voice Admit | [RFC5865] |
+----+----------------------+-----------+
| LE | Lower Effort | [RFC8622] |
+----+----------------------+-----------+

Table 2: Abbreviations for DSCPs and
PHB Groups

Table 2 summarizes the DSCP abbreviations used in currently published
RFCs, [RFC2474] [RFC2597] [RFC3246] [RFC5865] [RFC8622], as described
in the IANA registry [DSCP-registry]. The Default PHB is defined in
Section 4.1 of [RFC2474]. This provides Best Effort (BE) forwarding,
and the recommended DSCP of '000000' (Section 4.2.2.1 of [RFC2474]).
This is the lowest value in the set of Class Selector (CS) DSCPs, and
hence is also known as "CS0" [DSCP-registry].

NOTE: [RFC4594] specified a now deprecated use of Class Selector 1
(CS1) as the codepoint for the Lower Effort PHB. [RFC8622] updated
[RFC4594] and [RFC8325] and obsoleted [RFC3662], assigning the LE
DSCP codepoint to the Lower Effort PHB.

The Diffserv architecture allows forwarding treatments to be
associated with each DSCP, and the RFC series describes some of these
as PHBs. Although DSCPs are intended to identify specific treatment
requirements, multiple DSCPs might also be mapped (aggregated) to the
same forwarding treatment. DSCPs can be mapped to Treatment
Aggregates (TAs) that might result in re-marking (e.g., [RFC5160]
suggests Meta-QoS-Classes to help enable deployment of standard end-
to-end QoS classes).

3.2. DSCPs Used for Network Control Traffic

Network control traffic is defined as packet flows that are essential
for stable operation of the administered network (see [RFC4594],
Section 3). The traffic consists of the network control service
class and the OAM service class. This traffic is marked with a value
from a set of CS DSCPs. This traffic is often a special case within
a provider network, and ingress traffic with these DSCP markings can
be re-marked.

DSCP CS2 is recommended for the OAM (Operations, Administration, and
Maintenance) service class (see [RFC4594], Section 3.3).

DSCP CS6 is recommended for local network control traffic. This
includes routing protocols and OAM traffic that are essential to
network operation administration, control, and management.
Section 3.2 of [RFC4594] recommends that "CS6 marked packet flows
from untrusted sources (for example, end user devices) SHOULD be
dropped or remarked at ingress to the Diffserv network".

DSCP CS7 is reserved for future use by network control traffic. "CS7
marked packets SHOULD NOT be sent across peering points" [RFC4594],
Section 3.1.

Section 4.2.2.2 of [RFC2474] recommends PHBs selected by CS6 and CS7
"MUST give packets a preferential forwarding treatment by comparison
to the PHB selected by codepoint '000000'".

At the time of writing, there is evidence to suggest CS6 is actively
used by network operators for control traffic. A study of traffic at
a large Internet Exchange showed around 40% of ICMP traffic carried
this mark [IETF115-IEPG]. Similarly, another study found many
routers re-mark all traffic, except for packets carrying a DSCP with
the format 0b11xxxx (i.e., setting the higher order bits to 0b11, see
Section 4.2.1 of this document).

4. Re-marking the DSCP

It is a feature of the Diffserv architecture that the Diffserv field
of packets can be re-marked at the Diffserv domain boundaries (see
Section 2.3.4.2 of [RFC2475]). A DSCP can be re-marked at the
ingress of a domain. This re-marking can change the DSCP value used
on the remainder of an IP path, or the network can restore the
initial DSCP marking at the egress of the domain. The Diffserv field
can also be re-marked based on common semantics and agreements
between providers at a Diffserv domain boundary. Furthermore,
[RFC2474] states that re-marking must occur when there is a
possibility of theft or denial-of-service attack.

A packet that arrives with a DSCP that is not associated with a PHB,
results in an "unknown DSCP." A node could receive a packet with an
"unexpected DSCP" due to misconfiguration, or because there is no
consistent policy in place. The treatment of packets that are marked
with an unknown or an unexpected DSCP at Diffserv domain boundaries
is determined by the policy for a Diffserv domain. If packets are
received that are marked with an unknown or an unexpected DSCP by a
Diffserv domain interior node, [RFC2474] recommends forwarding the
packet using a default (Best Effort) treatment but without changing
the DSCP. This seeks to support incremental Diffserv deployment in
existing networks as well as preserve DSCP markings by routers that
have not been configured to support Diffserv (see also Section 4.3).
[RFC3260] clarifies that this re-marking specified by [RFC2474] is
intended for interior nodes within a Diffserv domain. For Diffserv
ingress nodes, the traffic conditioning required by [RFC2475] applies
first.

Reports measuring existing deployments have defined a set of
categories for DSCP re-marking [Cus17] [Bar18] in the following seven
observed re-marking behaviors:

Bleach-DSCP: bleaches all traffic (sets the DSCP to zero)

Bleach-ToS-Precedence: set the first three bits of the DSCP field to
0b000 (reset the three bits of the former ToS Precedence field,
defined in [RFC0791] and clarified in [RFC1122])

Bleach-some-ToS: set the first three bits of the DSCP field to 0b000
(reset the three bits of the former ToS Precedence field), unless
the first two bits of the DSCP field are 0b11

Re-mark-ToS: set the first three bits of the DSCP field to any value
different from 0b000 (replace the three bits of the former ToS
Precedence field)

Bleach-low: set the last three bits of the DSCP field to 0b000

Bleach-some-low: set the last three bits of the DSCP field to 0b000,
unless the first two bits of the DSCP field are 0b11

Re-mark-DSCP: re-marks all traffic to one or more particular (non-
zero) DSCP values

These behaviors are explained in the following subsections and cross-
referenced in the remainder of the document.

The network nodes forming a particular path might or might not have
supported Diffserv. It is not generally possible for an external
observer to determine which mechanism results in a specific re-
marking solely from the change in an observed DSCP value.

NOTE: More than one mechanism could result in the same DSCP re-
marking (see below). These behaviors were measured on both IPv4 and
IPv6 Internet paths between 2017 and 2021 [Cus17]. IPv6 routers were
found to perform all the types of re-marking described above to a
lesser extent than IPv4 ones.

4.1. Bleaching the DSCP Field

A specific form of re-marking occurs when the Diffserv field is re-
assigned to the default treatment: CS0 (0x00). This results in
traffic being forwarded using the BE PHB. For example, AF31 (0x1a)
would be bleached to CS0.

A survey reported that resetting all the bits of the Diffserv field
to 0 was seen to be more prevalent at the edge of the network and
rather less common in core networks [Cus17].

4.2. IP Type of Service Manipulations

The IETF first defined ToS precedence for IP packets in [RFC0791] and
updated it to be part of the ToS field in [RFC1349]. Since 1998,
this practice has been deprecated by [RFC2474]. [RFC2474] defines
DSCPs 0bxxx000 as the Class Selector codepoints, where PHBs selected
by these codepoints MUST meet the "Class Selector PHB Requirements"
described in Section 4.2.2.2 of [RFC2474].

A recent survey reports practices based on ToS semantics have not yet
been eliminated from the Internet and need to still be considered
when making new DSCP assignments [Cus17].

4.2.1. Impact of ToS Precedence Bleaching

Bleaching of the ToS Precedence field (see Section 4) resets the
first three bits of the DSCP field to zero (the former ToS Precedence
field), leaving the last three bits unchanged (see Section 4.2.1 of
[RFC2474]). A Diffserv node can be configured in a way that results
in this re-marking. This re-marking can also occur when packets are
processed by a router that is not configured with Diffserv (e.g.,
configured to operate on the former ToS Precedence field [RFC0791]).
At the time of writing, this is a common manipulation of the Diffserv
field. The following Figure depicts this re-marking.

+-+-+-+-+-+-+
|5|4|3|2|1|0|
+-+-+-+-+-+-+
|0 0 0|x x x|
+-+-+-+-+-+-+

Figure 2: Bits in the Diffserv Field Modified by Bleaching of the ToS
Precedence

Figure 2 shows bleaching of the ToS Precedence (see Section 4), based
on Section 3 of [RFC1349]. The bit positions marked 'x' are not
changed.

+--------+------+---------+----+---------+----+---------+----+
| 56/CS7 | 57 | 58 | 59 | 60 | 61 | 62 | 63 |
+--------+------+---------+----+---------+----+---------+----+
| 48/CS6 | 49 | 50 | 51 | 52 | 53 | 54 | 55 |
+--------+------+---------+----+---------+----+---------+----+
| 40/CS5 | 41 | 42 | 43 | 44/VA | 45 | 46/EF | 47 |
+--------+------+---------+----+---------+----+---------+----+
| 32/CS4 | 33 | 34/AF41 | 35 | 36/AF42 | 37 | 38/AF43 | 39 |
+--------+------+---------+----+---------+----+---------+----+
| 24/CS3 | 25 | 26/AF31 | 27 | 28/AF32 | 29 | 30/AF33 | 31 |
+--------+------+---------+----+---------+----+---------+----+
| 16/CS2 | 17 | 18/AF21 | 19 | 20/AF22 | 21 | 22/AF23 | 23 |
+--------+------+---------+----+---------+----+---------+----+
| 8/CS1 | 9 | 10/AF11 | 11 | 12/AF12 | 13 | 14/AF13 | 15 |
+========+======+=========+====+=========+====+=========+====+
| 0/CS0 | 1/LE | 2 | 3 | 4 | 5 | 6 | 7 |
+========+======+=========+====+=========+====+=========+====+

Table 3: DSCP Values

As a result of ToS Precedence Bleaching, each of the DSCPs in a
column are re-marked to the smallest DSCP in that column. Therefore,
the DSCPs in the bottom row have higher survivability across an end-
to-end Internet path.

Data on the observed re-marking at the time of writing was presented
in [IETF115-IEPG].

+========+=======+===============+======+===+===+===========+======+
| 0/CS0 | 1/LE | 2 | 3 | 4 | 5 | 6 | 7 |
+========+=======+===============+======+===+===+===========+======+
| Assigned | Re-marked | EXP/ | * | | Re-marked | EXP/ |
| | from AF11..41 | LU | | | from | LU |
| | | | | | AF13..EF | |
+----------------+---------------+------+---+---+-----------+------+

Table 4: 0b000xxx DSCPs

* DSCP 4 has been historically used by the SSH application [Kol10]

Table 4 shows 0b000xxx DSCPs. This highlights any current
assignments and whether they are affected by any known re-marking
behaviors, such as ToS Precedence Bleaching.

DSCPs of the form 0b000xxx can be impacted by known re-marking
behaviors of other assigned DSCPs. For example, ToS Precedence
Bleaching of popular DSCPs AF11, AF21, AF31, and AF41 would result in
traffic being re-marked with DSCP 2 in the Internet core. The Lower
Effort (LE) Per-Hop Behavior PHB uses a DSCP of 1. The DSCP value of
4 has been historically used by the SSH application, following
semantics that precede Diffserv [Kol10].

Bleach-ToS-Precedence (see Section 4) of packets with a DSCP 'x'
results in the DSCP being re-marked to 'x' & 0x07 and then forwarded
using the PHB specified for the resulting DSCP in that Diffserv
domain. In subsequent networks, the packet will receive treatment as
specified by the domain's operator corresponding to the re-marked
codepoint.

A variation of this observed re-marking behavior clears the top three
bits of a DSCP, unless these have values 0b110 or 0b111
(corresponding to the CS6 and CS7 DSCPs). As a result, a DSCP value
greater than 48 decimal (0x30) is less likely to be impacted by ToS
Precedence Bleaching.

4.2.2. Impact of ToS Precedence Re-marking

[RFC2474] states:

| Implementors should note that the DSCP field is six bits wide.
| DS-compliant nodes MUST select PHBs by matching against the entire
| 6-bit DSCP field, e.g., by treating the value of the field as a
| table index which is used to select a particular packet handling
| mechanism which has been implemented in that device.

This replaced re-marking according to ToS precedence (see Section 4)
[RFC1349]. These practices based on ToS semantics have not yet been
eliminated from deployed networks.

+-+-+-+-+-+-+
|5|4|3|2|1|0|
+-+-+-+-+-+-+
|0 0 1|x x x|
+-+-+-+-+-+-+

Figure 3: Bits in the Diffserv Field Modified by ToS Precedence
Re-marking

Figure 3 shows the ToS Precedence Re-marking (see Section 4) observed
behavior, based on Section 3 of [RFC1349]. The bit positions marked
'x' remain unchanged.

A less common re-marking, ToS Precedence Re-marking sets the first
three bits of the DSCP to a non-zero value corresponding to a CS PHB.
This re-marking occurs when routers are not configured to perform
Diffserv re-marking.

If ToS Precedence Re-marking occurs, packets are forwarded using the
PHB specified for the resulting DSCP in that domain. For example,
the AF31 DSCP (0x1a) could be re-marked to either AF11 or AF21. If
such a re-marked packet further traverses other Diffserv domains, it
would receive treatment as specified by each domain's operator
corresponding to the re-marked codepoint.

4.3. Re-marking to a Particular DSCP

A network device might re-mark the Diffserv field of an IP packet
based on a local policy with a specific set of DSCPs (see Section 4).

Section 3 of [RFC2474] recommends: "Packets received with an
unrecognized codepoint SHOULD be forwarded as if they were marked for
the Default behavior, and their codepoints should not be changed."
Some networks might not follow this recommendation and instead re-
mark packets with these codepoints to the default class: CS0 (0x00).
This re-marking is sometimes performed using a Multi-Field (MF)
classifier [RFC2475] [RFC3290] [RFC4594].

If re-marking occurs, packets are forwarded using the PHB specified
for the resulting DSCP in that domain. As an example, re-marking
traffic AF31, AF32, and AF33 all to a single DSCP, e.g., AF11, stops
any drop probability differentiation, which may have been expressed
by these three DSCPs. If such a re-marked packet further traverses
other domains, it would receive treatment as specified by the
domain's operator corresponding to the re-marked codepoint.
Bleaching (see Section 4) is a specific example of this observed re-
marking behavior that re-marks to CS0 (0x00) (see Section 4.1).

5. Interpretation of the IP DSCP at Lower Layers

Transmission systems and subnetworks can, and do, utilize the
Diffserv field in an IP packet to set a QoS-related field or function
at the lower layer. A lower layer could also implement a traffic-
conditioning function that could re-mark the DSCP used at the IP
layer. This function is constrained by designs that utilize fewer
than 6 bits to express the service class and, therefore, infer a
mapping to a smaller L2 QoS field, for example, the 3-bit Priority
Code Point (PCP) field in an IEEE Ethernet 802.1Q header, the 3-bit
User Priority (UP) field, or the 3-bit Traffic Class field of Multi-
Protocol Label Switching (MPLS). A Treatment Aggregate (TA)
[RFC5127] is an optional intermediary mapping group of BAs to PHBs.

5.1. Mapping Specified for IEEE 802

The IEEE specifies standards that include mappings for DSCPs to lower
layer elements.

5.1.1. Mapping Specified for IEEE 802.1

IEEE 802.1Q specified a 3-bit PCP field, which includes a tag that
allows Ethernet frames to be marked as one of eight priority values
[IEEE-802.1Q]. Use of this field is described by various documents,
including IEEE P802.1p and IEEE 802.1D.

The mapping specified in [IEEE-802.1Q] revises a previous standard,
[IEEE-802.1D], in an effort to align with Diffserv practice
[RFC4594]. In 802.1Q, the traffic types are specified to match the
first three bits of a suitable DSCP (e.g., the first three bits of
the Expedited Forwarding (EF) DSCP are mapped to a PCP of 5).

In this mapping, PCP0 is used to indicate the default Best Effort
treatment, and PCP1 indicates a background traffic class. The
remaining PCP values indicate increasing priority. Internet control
traffic can be marked as CS6, and network control is marked as CS7.

Other re-marking behaviors have also been implemented in Ethernet
equipment. Historically, a previous standard, [IEEE-802.1D], used
both PCP1 (Background) and PCP2 (Spare) to indicate lower priority
than PCP0, and some other devices do not assign a lower priority to
PCP1.

5.1.2. Mapping Specified for IEEE 802.11

Section 6 of [RFC8325] provides a brief overview of IEEE 802.11 QoS.
The IEEE 802.11 standards [IEEE-802.11] provide Media Access Control
(MAC) functions to support QoS in WLANs using Access Categories
(ACs). The upstream UP in the 802.11 header has a 3-bit QoS value.
A DSCP can be mapped to the UP. [RFC8622] added a mapping for the LE
DSCP to AC_BK (Background).

Most current Wi-Fi implementations use a default mapping that maps
the first three bits of the DSCP to the 802.11 UP value. This is an
example of equipment still classifying on ToS Precedence (which could
be seen as a simple method to map IP layer Diffserv to layers
offering only 3-bit QoS codepoints). Then, in turn, this is mapped
to the four Wi-Fi Multimedia (WMM) Access Categories. The Wi-Fi
Alliance has also specified a more flexible mapping that follows
[RFC8325] and provides functions at an Access Point (AP) to re-mark
packets as well as a QoS Map that maps each DSCP to an AC
[WIFI-ALLIANCE].

+-+-+-+-+-+-+
|5|4|3|2|1|0|
+-+-+-+-+-+-+
|x x x|. . .|
+-+-+-+-+-+-+

Figure 4: DSCP Bits Commonly Mapped to the UP in 802.11

The bit positions marked 'x' are mapped to the 3-bit UP value, while
the ones marked '.' are ignored.

[RFC8325] notes inconsistencies that can result from such re-marking
and recommends a different mapping to perform this re-marking,
dependent on the direction in which a packet is forwarded. It
provides recommendations for mapping a DSCP to an IEEE 802.11 UP for
interconnection between wired and wireless networks. The
recommendation in Section 5.1.2 maps network control traffic, CS6 and
CS7, as well as unassigned DSCPs, to UP 0 when forwarding in the
upstream direction (wireless-to-wired). It also recommends mapping
CS6 and CS7 traffic to UP 7 when forwarding in the downstream
direction (Section 4.1 of [RFC8325]).

For other UPs, [RFC8325] recommends a mapping in the upstream
direction (wireless-to-wired interconnections) that derives the DSCP
from the value of the UP multiplied by 8. This mapping, currently
used by some Access Points (APs), can result in a specific DSCP re-
marking behavior:

| wherein DSCP values are derived from UP values by multiplying the
| UP values by 8 (i.e., shifting the three UP bits to the left and
| adding three additional zeros to generate a DSCP value). This
| derived DSCP value is then used for QoS treatment between the
| wireless AP and the nearest classification and marking policy
| enforcement point (which may be the centralized wireless LAN
| controller, relatively deep within the network). Alternatively,
| in the case where there is no other classification and marking
| policy enforcement point, then this derived DSCP value will be
| used on the remainder of the Internet path.

This can result in re-marking by Bleach-low (see Section 4).

+-+-+-+-+-+-+
|5|4|3|2|1|0|
+-+-+-+-+-+-+
|x x x|0 0 0|
+-+-+-+-+-+-+

Figure 5: Bits in the Diffserv Field Modified by Re-marking
Observed as a Result of UP-to-DSCP Mapping in Some 802.11
Networks

An alternative to UP-to-DSCP remapping uses the DSCP value of a
downstream IP packet (e.g., the Control and Provisioning of Wireless
Access Points, CAPWAP, protocol [RFC5415] maps an IP packet Diffserv
field to the Diffserv field of the outer IP header in a CAPWAP
tunnel).

Some current constraints of Wi-Fi mapping are discussed in Section 2
of [RFC8325]. A QoS profile can be used to limit the maximum DSCP
value used for the upstream and downstream traffic.

5.2. Diffserv and MPLS

Multi-Protocol Label Switching (MPLS) specified eight MPLS Traffic
Classes (TCs), which restrict the number of different treatments
[RFC5129]. [RFC5127] describes the aggregation of Diffserv service
classes and introduces four Diffserv TAs. Traffic marked with
multiple DSCPs can be forwarded in a single MPLS TC.

There are three Label Switching Router (LSR) models: the Pipe, the
Short Pipe, and the Uniform Model [RFC3270]. In the Uniform and Pipe
models, the egress MPLS router forwards traffic based on the received
MPLS TC. The Uniform Model includes an egress DSCP rewrite. With
the Short Pipe Model, the egress MPLS router forwards traffic based
on the Diffserv DSCP as present at the egress router. If the domain
supports IP and MPLS QoS differentiation, controlled behavior
requires the DSCP of an (outer) IP header to be assigned or re-
written by all domain ingress routers to conform with the domain's
internal Diffserv deployment. Note that the Short Pipe Model is
prevalent in MPLS domains.

5.2.1. Mapping Specified for MPLS

[RFC3270] defines a flexible solution for support of Diffserv over
MPLS networks. This allows an MPLS network administrator to select
how BAs (marked by DSCPs) are mapped onto Label Switched Paths (LSPs)
to best match the Diffserv, Traffic Engineering, and protection
objectives within their particular network.

Mappings from the IP DSCP to the MPLS header are defined in
Section 4.2 of [RFC3270].

The Pipe Model conveys the "LSP Diff-Serv Information" to the LSP
Egress so that its forwarding treatment can be based on the IP DSCP.

When Penultimate Hop Popping (PHP) is used, the Penultimate LSR needs
to be aware of the encapsulation mapping for a PHB to the label
corresponding to the exposed header to perform Diffserv Information
Encoding (Section 2.5.2 of [RFC3270]).

5.2.2. Mapping Specified for MPLS Short Pipe

The Short Pipe Model is an optional variation of the Pipe Model
[RFC3270].

ITU-T Y.1566 [ITU-T-Y.1566] further defined a set of four common QoS
classes and four auxiliary classes to which a DSCP can be mapped when
interconnecting Ethernet, IP, and MPLS networks. [RFC8100] describes
four TAs for interconnection with four defined DSCPs. This was
motivated by the requirements of MPLS network operators that use
Short Pipe tunnels but can be applicable to other networks, both MPLS
and non-MPLS.

[RFC8100] recommends preserving the notion of end-to-end service
classes and recommends a set of standard DSCPs mapped to a small set
of standard PHBs at interconnection. The key requirement is that the
DSCP at the network ingress is restored at the network egress. The
current version of [RFC8100] limits the number of DSCPs to 6, and 3
more are suggested for extension. [RFC8100] respects the deployment
of PHB groups for DS domain-internal use, which limits the number of
acceptable external DSCPs (and possibilities for their transparent
transport or restoration at network boundaries). In this design,
packets marked with DSCPs not part of the codepoint scheme [RFC8100]
are treated as unexpected and will possibly be re-marked (a Re-mark-
DSCP, see Section 4 behavior) or dealt with via additional agreements
among the operators of the interconnected networks. [RFC8100] can be
extended to support up to 32 DSCPs by future standards. [RFC8100] is
operated by at least one Tier 1 backbone provider. Use of the MPLS
Short Pipe Model favors re-marking unexpected DSCP values to zero in
the absence of additional agreements, as explained in [RFC8100].
This can result in bleaching (see Section 4).

+=======================================+==========+
| Treatment Aggregate [RFC8100] | DSCP |
+=======================================+==========+
| Telephony Service Treatment Aggregate | EF |
| | VA |
+---------------------------------------+----------+
| Bulk Real-Time Treatment Aggregate | AF41 |
| | (AF42)* |
| | (AF43)* |
+---------------------------------------+----------+
| Assured Elastic Treatment Aggregate | AF31 |
| | AF32 |
| | (AF33)** |
+---------------------------------------+----------+
| Default / Elastic Treatment Aggregate | BE/CS0 |
+---------------------------------------+----------+
| Network Control: Local Use (LU) | CS6 |
+---------------------------------------+----------+

Table 5: The Short Pipe MPLS Mapping from [RFC8100]

* May be added

** Reserved for the extension of PHBs

5.3. Mapping Specified for Mobile Networks

Mobile LTE and 5G standards have evolved from older Universal Mobile
Telecommunications System (UMTS) standards and support Diffserv. LTE
(4G) and 5G standards [SA-5G] identify traffic classes at the
interface between User Equipment (UE) and the mobile Packet Core
network by QCI (QoS Class Identifiers) and 5QI (5G QoS Identifier).
The 3GPP standards do not define or recommend any specific mapping
between each QCI or 5QI and Diffserv (and mobile QCIs are based on
several criteria service class definitions). The way packets are
mapped at the Packet Data Network Gateway (P-GW) boundary is
determined by network operators. However, TS 23.107 (version 16.0.0,
applies to LTE and below) mandates that Differentiated Services,
defined by the IETF, shall be used to interoperate with IP backbone
networks.

The GSM Association (GSMA) has defined four aggregated classes and
seven associated PHBs in their guidelines for IP Packet eXchange
(IPX) Provider networks [GSMA-IR.34]. This was previously specified
as the "Inter-Service Provider IP Backbone Guidelines" and provides a
mobile ISP to ISP QoS mapping mechanism and interconnection with
other IP networks in the general Internet. If provided an IP VPN,
the operator is free to apply its DS domain-internal codepoint scheme
at outer headers and inner IPX DSCPs may be transported
transparently. The guidelines also describe a case where the DSCP
marking from a Service Provider cannot be trusted (depending on the
agreement between the Service Provider and its IPX Provider). In
this situation, the IPX Provider can re-mark the DSCP value to a
static default value.

+====================================+======+
| QoS Class in [GSMA-IR.34] | PHB |
+====================================+======+
| Conversational | EF |
+------------------------------------+------+
| Streaming | AF41 |
+------------------------------------+------+
| Interactive | AF31 |
| +------+
| (ordered by priority, AF3 highest) | AF32 |
| +------+
| | AF21 |
| +------+
| | AF11 |
+------------------------------------+------+
| Background | CS0 |
+------------------------------------+------+

Table 6: The PHB Mapping Recommended in
the Guidelines Recommended in
[GSMA-IR.34]

5.4. Mapping Specified for Carrier Ethernet

MEF Forum (MEF) provides a mapping of DSCPs at the IP layer to
quality of service markings in the Ethernet frame headers [MEF-23.1].

5.5. Re-marking as a Side Effect of Another Policy

This includes any other re-marking that does not happen as a result
of traffic conditioning, such as policies and L2 procedures that
result in re-marking traffic as a side effect of other functions
(e.g., in response to Distributed Denial of Service, DDoS).

5.6. Summary

This section has discussed the various ways in which DSCP re-marking
behaviors can arise from interactions with lower layers.

A provider service path may consist of sections where multiple and
changing layers use their own code points to determine differentiated
forwarding (e.g., IP to MPLS to IP to Ethernet to Wi-Fi).

6. Considerations for DSCP Selection

This section provides advice for the assignment of a new DSCP value.
It is intended to aid the IETF and IESG in considering a request for
a new DSCP. This section identifies known issues that might
influence the finally assigned DSCP and provides a summary of
considerations for assignment of a new DSCP.

6.1. Effect of Bleaching and Re-marking to a Single DSCP

Section 4 describes re-marking of the DSCP. New DSCP assignments
should consider the impact of bleaching or re-marking (see Section 4)
to a single DSCP, which can limit the ability to provide the expected
treatment end-to-end. This is particularly important for cases where
the codepoint is intended to result in lower than Best Effort
treatment, as was the case when defining the LE PHB [RFC8622].
Forwarding LE using the default PHB is in line with [RFC8622], but it
is recommended to maintain the distinct LE DSCP codepoint end-to-end
to allow for differentiated treatment by domains supporting LE.
Rewriting the LE DSCP to the default class (CS0) results in an
undesired promotion of the priority for LE traffic in such a domain.
Bleaching the lower three bits of the DSCP (both Bleach-low and
Bleach-some-low in Section 4), as well as re-marking to a particular
DSCP, can result in similar changes of priority relative to traffic
that is marked with other DSCPs.

6.2. Where the Proposed DSCP > 0x07 (7)

This considers a proposed DSCP with a codepoint larger than 7.

Although the IETF specifications require systems to use DSCP marking
semantics in place of methods based on the former ToS field, the
current recommendation is that any new assignment where the DSCP is
greater than 0x07 should consider the semantics associated with the
resulting DSCP when the ToS Precedence is bleached (Bleach-ToS-
Precedence and Bleach-some-ToS, Section 4) or ToS Precedence Re-
marking (Re-mark-ToS, Section 4) is experienced. For example, it can
be desirable to avoid choosing a DSCP that could be re-marked to LE,
Lower Effort [RFC8622], which could otherwise potentially result in a
priority inversion in the treatment.

6.2.1. Where the Proposed DSCP&0x07=0x01

This considers a proposed DSCP where the least significant 3 bits
together represent a value of 1 (i.e., 0b001).

As a consequence of assigning the LE PHB [RFC8622], the IETF
allocated the DSCP 0b000001 from Pool 3.

When making assignments where the DSCP has a format "0bxxx001", the
case of Bleach-ToS-Precedence (Section 4) of a DSCP to a value of
0x01 needs to be considered. ToS Precedence Bleaching will result in
demoting the traffic to the Lower Effort PHB. Care should be taken
to consider the implications of re-marking when choosing to assign a
DSCP with this format.

6.3. Where the Proposed DSCP <= 0x07 (7)

This considers a proposed DSCP where the DSCP is less than or equal
to 7.

ToS Precedence Bleaching or ToS Precedence Re-marking can
unintentionally result in extra traffic aggregated to the same DSCP.
For example, after experiencing ToS Precedence Bleaching, all traffic
marked AF11, AF21, AF31, and AF41 would be aggregated with traffic
marked with DSCP 2 (0x02), increasing the volume of traffic that
receives the treatment associated with DSCP 2. New DSCP assignments
should consider unexpected consequences arising from this observed
re-marking behavior.

6.4. Impact on Deployed Infrastructure

Behavior of deployed PHBs and conditioning treatments also needs to
be considered when assigning a new DSCP. Network operators have
choices when it comes to configuring Diffserv support within their
domains, and the observed re-marking behaviors described in the
previous section can result from different configurations and
approaches:

Networks not re-marking Diffserv:
A network that either does not implement PHBs or implements one or
more PHBs while restoring the DSCP field at network egress with
the value at network ingress. Operators in this category pass all
DSCPs transparently.

Networks that condition the DSCP:
A network that implements more than one PHB and enforces Service
Level Agreements (SLAs) with its peers. Operators in this
category use conditioning to ensure that only traffic that matches
a policy is permitted to use a specific DSCP (see [RFC8100]).
Operators need to classify the received traffic, assign it to a
corresponding PHB, and could re-mark the DSCP to a value that is
appropriate for the domain's deployed Diffserv infrastructure.

Networks that re-mark in some other way, which includes:
1. Networks containing misconfigured devices that do not comply
with the relevant RFCs.

2. Networks containing devices that implement an obsolete
specification or contain software bugs.

3. Networks containing devices that re-mark the DSCP as a result
of lower layer interactions.

The DSCP re-marking corresponding to the Bleach-ToS-Precedence
(Section 4) observed behavior can arise for various reasons, one of
which is old equipment that precedes Diffserv. The same re-marking
can also arise in some cases when traffic conditioning is provided by
Diffserv routers at operator boundaries or as a result of
misconfiguration.

6.5. Considerations to Guide the Discussion of a Proposed New DSCP

A series of questions emerge that need to be answered when
considering a proposal to the IETF that requests a new assignment.
These questions include:

* Is the request for Local Use within a Diffserv domain that does
not require interconnection with other Diffserv domains? This
request can use DSCPs in Pool 2 for Local or Experimental Use,
without any IETF specification for the DSCP or associated PHB.

* What are the characteristics of the proposed service class? What
are the characteristics of the traffic to be carried? What are
the expectations for treatment?

* Service classes [RFC4594] that can utilize existing PHBs should
use assigned DSCPs to mark their traffic: Could the request be met
by using an existing IETF DSCP?

* Specification of a new recommended DSCP requires Standards Action.
[RFC2474] states: "Each standardized PHB MUST have an associated
RECOMMENDED codepoint". If approved, new IETF assignments are
normally made by IANA in Pool 1, but the IETF can request
assignments to be made from Pool 3 [RFC8436]. Does the Internet
Draft contain an appropriate request to IANA?

* The value selected for a new DSCP can impact the ability of an
operator to apply logical functions (e.g., a bitwise mask) to
related codepoints when configuring Diffserv. A suitable value
can simplify configurations by aggregating classification on a
range of DSCPs. This classification based on DSCP ranges can
increase the comprehensibility of documenting forwarding
differentiation.

* Section 5.2 describes examples of treatment aggregation. What are
the effects of treatment aggregation on the proposed DSCP?

* Section 5 describes some observed treatments by layers below IP.
What are the implications of the treatments and mapping described
in Section 5 on the proposed DSCP?

* DSCPs are assigned to PHBs and can be used to enable nodes along
an end-to-end path to classify the packet for a suitable PHB.
Section 4 describes some observed re-marking behavior, and
Section 6.4 identifies root causes for why this re-marking is
observed. What is the expected effect of currently-deployed re-
marking on the service, end-to-end or otherwise?

7. IANA Considerations

IANA has added the following text as a note at the top of the
"Differentiated Services Field Codepoints (DSCP)" registry
[DSCP-registry]: "See RFC 9435 for considerations when assigning a
new codepoint from the DSCP registry."

8. Security Considerations

The security considerations are discussed in the security
considerations of each cited RFC.

9. References

9.1. Normative References

[DSCP-registry]
IANA, "Differentiated Services Field Codepoints (DSCP)",
<https://www.iana.org/assignments/dscp-registry/>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.

[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.

[RFC3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, DOI 10.17487/RFC3260, April 2002,
<https://www.rfc-editor.org/info/rfc3260>.

[RFC3290] Bernet, Y., Blake, S., Grossman, D., and A. Smith, "An
Informal Management Model for Diffserv Routers", RFC 3290,
DOI 10.17487/RFC3290, May 2002,
<https://www.rfc-editor.org/info/rfc3290>.

[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<https://www.rfc-editor.org/info/rfc4594>.

[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January
2008, <https://www.rfc-editor.org/info/rfc5129>.

[RFC8100] Geib, R., Ed. and D. Black, "Diffserv-Interconnection
Classes and Practice", RFC 8100, DOI 10.17487/RFC8100,
March 2017, <https://www.rfc-editor.org/info/rfc8100>.

[RFC8436] Fairhurst, G., "Update to IANA Registration Procedures for
Pool 3 Values in the Differentiated Services Field
Codepoints (DSCP) Registry", RFC 8436,
DOI 10.17487/RFC8436, August 2018,
<https://www.rfc-editor.org/info/rfc8436>.

9.2. Informative References

[AX.25-over-IP]
Learmonth, I. R., "Internet Protocol Encapsulation of
AX.25 Frames", Work in Progress, Internet-Draft, draft-
learmonth-intarea-rfc1226-bis-03, 23 May 2021,
<https://datatracker.ietf.org/doc/html/draft-learmonth-
intarea-rfc1226-bis-03>.

[Bar18] Barik, R., Welzl, M., Elmokashfi, A., Dreibholz, T., and
S. Gjessing, "Can WebRTC QoS Work? A DSCP Measurement
Study", 2018 30th International Teletraffic Congress (ITC
30), DOI 10.1109/ITC30.2018.00034, September 2018,
<https://doi.org/10.1109/ITC30.2018.00034>.

[Cus17] Custura, A., Venne, A., and G. Fairhurst, "Exploring DSCP
modification pathologies in mobile edge networks", 2017
Network Traffic Measurement and Analysis Conference (TMA),
DOI 10.23919/TMA.2017.8002923, June 2017,
<https://doi.org/10.23919/TMA.2017.8002923>.

[GSMA-IR.34]
GSM Association, "Guidelines for IPX Provider networks
(Previously Inter-Service Provider IP Backbone
Guidelines)", Version 17.0, IR.34, May 2021,
<https://www.gsma.com/newsroom/wp-content/uploads/
IR.34-v17.0.pdf>.

[IEEE-802.11]
IEEE, "IEEE Standard for Information Technology -
Telecommunications and Information Exchange Between
Systems - Local and Metropolitan Area Networks - Specific
Requirements - Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications",
DOI 10.1109/IEEESTD.2021.9363693, IEEE Standard 802.11,
February 2021,
<https://standards.ieee.org/ieee/802.11/7028/>.

[IEEE-802.1D]
IEEE, "IEEE Standard for Local and metropolitan area
network--Media Access Control (MAC) Bridges", IEEE
Standard 802.1D-2004, DOI 10.1109/IEEESTD.2004.94569, June
2004, <https://doi.org/10.1109/IEEESTD.2004.94569>.

[IEEE-802.1Q]
IEEE, "IEEE Standard for Local and Metropolitan Area
Network--Bridges and Bridged Networks", IEEE Standard
802.1Q-2018, DOI 10.1109/IEEESTD.2018.8403927, July 2018,
<https://doi.org/10.1109/IEEESTD.2018.8403927>.

[IETF115-IEPG]
Custura, A., "Real-world DSCP Traversal Measurements",
November 2022,
<https://datatracker.ietf.org/meeting/115/materials/
slides-115-iepg-sessa-considerations-for-assigning-dscps-
01>.

[ITU-T-Y.1566]
ITU-T Recommendation, "Quality of service mapping and
interconnection between Ethernet, Internet Protocol and
multiprotocol label switching networks", ITU-T Y.1566,
July 2012, <https://www.itu.int/rec/T-REC-Y.1566/en>.

[Kol10] Kolu, A., "Subject: bogus DSCP value for ssh", message to
the freebsd-stable mailing list, 12 July 2010,
<https://lists.freebsd.org/pipermail/freebsd-
stable/2010-July/057710.html>.

[MEF-23.1] MEF, "Implementation Agreement MEF 23.1 Carrier Ethernet
Class of Service - Phase 2", MEF 23.1, January 2012,
<https://mef.net/Assets/Technical_Specifications/PDF/
MEF_23.1.pdf>.

[NQB-PHB] White, G. and T. Fossati, "A Non-Queue-Building Per-Hop
Behavior (NQB PHB) for Differentiated Services", Work in
Progress, Internet-Draft, draft-ietf-tsvwg-nqb-18, 10 July
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
tsvwg-nqb-18>.

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.

[RFC1349] Almquist, P., "Type of Service in the Internet Protocol
Suite", RFC 1349, DOI 10.17487/RFC1349, July 1992,
<https://www.rfc-editor.org/info/rfc1349>.

[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC2597, June 1999,
<https://www.rfc-editor.org/info/rfc2597>.

[RFC3086] Nichols, K. and B. Carpenter, "Definition of
Differentiated Services Per Domain Behaviors and Rules for
their Specification", RFC 3086, DOI 10.17487/RFC3086,
April 2001, <https://www.rfc-editor.org/info/rfc3086>.

[RFC3246] Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le
Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
<https://www.rfc-editor.org/info/rfc3246>.

[RFC3270] Le Faucheur, F., Ed., Wu, L., Davie, B., Davari, S.,
Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen,
"Multi-Protocol Label Switching (MPLS) Support of
Differentiated Services", RFC 3270, DOI 10.17487/RFC3270,
May 2002, <https://www.rfc-editor.org/info/rfc3270>.

[RFC3662] Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
Per-Domain Behavior (PDB) for Differentiated Services",
RFC 3662, DOI 10.17487/RFC3662, December 2003,
<https://www.rfc-editor.org/info/rfc3662>.

[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
Diffserv Service Classes", RFC 5127, DOI 10.17487/RFC5127,
February 2008, <https://www.rfc-editor.org/info/rfc5127>.

[RFC5160] Levis, P. and M. Boucadair, "Considerations of Provider-
to-Provider Agreements for Internet-Scale Quality of
Service (QoS)", RFC 5160, DOI 10.17487/RFC5160, March
2008, <https://www.rfc-editor.org/info/rfc5160>.

[RFC5415] Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
Ed., "Control And Provisioning of Wireless Access Points
(CAPWAP) Protocol Specification", RFC 5415,
DOI 10.17487/RFC5415, March 2009,
<https://www.rfc-editor.org/info/rfc5415>.

[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, DOI 10.17487/RFC5865, May 2010,
<https://www.rfc-editor.org/info/rfc5865>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8325] Szigeti, T., Henry, J., and F. Baker, "Mapping Diffserv to
IEEE 802.11", RFC 8325, DOI 10.17487/RFC8325, February
2018, <https://www.rfc-editor.org/info/rfc8325>.

[RFC8622] Bless, R., "A Lower-Effort Per-Hop Behavior (LE PHB) for
Differentiated Services", RFC 8622, DOI 10.17487/RFC8622,
June 2019, <https://www.rfc-editor.org/info/rfc8622>.

[SA-5G] 3GPP, "System architecture for the 5G System (5GS)",
TS 23.501, 2019.

[WIFI-ALLIANCE]
Wi-Fi Alliance, "Wi-Fi QoS Management Specification
Version 2.0", 2021.

Acknowledgements

The authors acknowledge the helpful discussions and analysis by Greg
White and Thomas Fossati in a draft concerning NQB. Ruediger Geib
and Brian Carpenter contributed comments, along with other members of
the TSVWG.

Authors' Addresses

Ana Custura
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: [email protected]

Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: [email protected]

Raffaello Secchi
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: [email protected]